The development of metal ink containing metal nano-particles has advantages in that a metal wiring having a fine pattern may be printed on various substrates without using a complicated process of photolithography, by only a single printing process such as a screen printing process, an inkjet printing process, a gravure offset printing process, a reverse offset printing process, or the like, to thereby simplify the process. In addition, manufacturing cost may be significantly decreased due to simplification of the process, and a high integration and high efficiency printed circuit may be manufactured due to fineness of a wiring width.
Particularly, in the case of a flexible printed circuit board in which a circuit is printed on a resin film, since the flexible board itself may be damaged while being subjected to a complicated series of processes accompanied with lithography, that is, an applying process, a drying process, an exposure process, an etching process, and a removal process, the metal ink containing mono-dispersed nano-particles capable of directly forming a circuit on the resin film has been urgently demanded.
Realistically, it is almost impossible to prepare metal particles having a nano size and oxidation stability using a method of preparing metal nano-particles according to the traditional physical method, and it is more difficult to provide a metal nano-particle dispersion solution having the oxidation stability by dispersing the metal particle in a polar solvent such as deionized water, or the like.
Generally, the metal nano-particle as described above has been synthesized by a wet reducing process, and a property in view of exhibiting conductivity was deteriorated due to a problem in that a surface oxide film is easily formed on the metal particle at the time of the synthesis.
As a method of preparing metal nano-particle having oxidation stability against a polar solvent according to the related art, research into a wet reducing method of preparing the metal nano-particle by reducing metal ions using a reducing agent in the presence of a surfactant solution and anti-oxidant (oxidation preventing agent) has been disclosed in Korean Patent Laid-Open Publication No. 2000-0018196 (Patent Document 1). This method, which is a method of preparing a small reactor having a nano size using a surfactant to adjust a size of the particle in the reactor by reduction reaction by the reducing agent, has advantages in that the size of the particle may be easily adjusted and the particle is stable, but there are problems such as a resistance increase due to the surfactant and the anti-oxidant used in order to secure oxidation stability and dispersion stability at the time of forming the wiring and a metal film, and the like.
In addition, a disadvantage in that a metal oxide film is formed on a surface of the produced nano-particle, which deteriorates physical properties such as conductivity of the metal, and the like, should still be addressed.
Further, a problem in that when the produced nano-particle is firmly immobilized on the substrate, in the case in which a binder is used at the time of forming the wiring and the metal film, the conductivity may not be sufficiently secured due to the resistance increase, which still should be solved.